Equator

An equator is the imaginary great circle line that passes across a planet's centre, and that is aligned with the planet's direction of rotation. As such an equator is perpendicular with the planet's axis of rotation, and it divides the planet into two equal northern and southern hemispheres. For these qualities, the line of the equator may be defined also as the linear area on a planet whose relationship with the Sun remains more or less constant through out the year, that is, regardless of the planet's location as it orbits the sun in the ecliptic. Thus, where areas farther distanced from the equator go through different seasons throughout the year as they become more inclined toward or away from the sun; the areas immediately close to the line of the equator experience no or little changes in temperature and duration of day and night through out the year, and therewith have no seasons.

Other planets and spherical astronomical bodies have equators similarly defined. The length of Earth's Equator is roughly 40,075 kilometres (24,901 mi).